Estimating the performance of supercharged piston aero engines

This is a topic that I have discussed to a small degree with Gruntguru in the recent past.

The discussion stemmed from a forum topic on a WW2 aircraft forum, th epremise being what if the Vulture was a success and had not been cancelled.

Some background.

The Rolls-Royce Vulture was a 24 cylinder supercharged aero engine, with its cylinder banks arranged in an X, with 90° between them. The bore and stroke were 5.0" x 5.5", the same as the earlier Kestrel and the later Kestrel development, the Peregrine. Often it is stated that the Vulture is 4 Peregrine cylinder banks arranged around a single crankshaft - this is not correct, the Vulture having a bore spacing of 6.1" against the Kestrel/Peregrine's 5.625" (RR Merlin 6.075" bore spacing, Allison V-1710 6.3").

The supercharger fitted was a single stage two speed unit with a 12" diameter centrifugal compressor. The compressor was later pressed into service as the first stage of the Merlin 60 series two stage supercharger, and would remain for most of the two stage Merlins.

The Vulture used a master rod and slave arrangement, so as to minimise the length of the engine. The design of the master rod would cause many of the problems that would haunt the Vulture program.

In typical Rolls-Royce fashion each of the issues would be identified and a program to find their solution was set in motion. At the time of cancellation in March 1941 all of the major issues are said to have been solved.

There were three Vulture variants under development at the time of cancellation.The first was the Vulture II, as fitted to the Avro Manchester. This proved to be extremely troublesome, with many engine failures and fires.

The other two variants were described as the "fighter" versions - the MkIV and MkV. The MkV was fitted to the Hawker Tornado prototype (almost identical to the Sabre powered Typhoon) and suffered very few problems during the flight test program.

Because of the issues the Vulture was having with reliability the operating limits for the engine were given as:Max climb - 30 minutes at 2850rpm, +6psi boostCruise in auto weak - continuous at 2600rpm, +2psi boost.

The question is, what do those limitations equate to in terms of horsepower?

Once Rolls-Royce achieved an acceptable level of reliability for its engines they then set about improving its performance. The Merlin, for example, went from a 1000hpp flat out for 5 minutes engine to one that could cruise continuously at 1250hp+, with maximum 5 minute ratings of over 2200hp. Much of the performance jump was to do with the quality of the fuels used - the Merlin III went from 1030hp to 1300hp with the change from 87 octane to 100 octane fuel. But the improvements would not have been possible without the strengthening of key components.

When I have estimated the performance of engines in the past I have generally taken the BMEP from an existing engine and applied it to the new engine.

For example:The Rolls-Royce Merlin XX with single stage two speed supercharger and PN100/130 fuel was rated for 1480hp @ 3000rpm - as used in the Lancaster.

This equates to a BMEP of 1634kPa/237psi.

Applying that BMEP to the Vulture gives 2210hp at the restricted maximum rpm of 2850, and 2480hp @ 3200rpm, the design rpm for the engine. The RRHT confirms that the Vulture tested several times at 2500hp. Lumsden, British Piston Engines and their Aircraft, says that the Vulture was tested to 3000hp, but the RRHT could not confirm this.

The Vulture and Merlin share the same combustion chamber shape - that is a flat chamber with 4 vertically mounted valves. The Merlin does, however, have the larger bore of 5.4" vs 5" for the Vulture. Does this make the use of the Merlin BMEP in the Vulture more valid?

Also, how much does the supercharger influence performance? The Allison V-1710 was similar in size to the Merlin, with a 5.5"bore giving around 1000cc/60ci extra displacement. It also had a pentroof combustion chamber. The V-1710 generally lagged behind the Merlin in power development. Allison fitted a V-1710 with a two stage supercharger from a Merlin 60-series and tested it on the bench. The result was almost identical power curves. That would suggest that the power potential of an engine is dominated by its supercharger.

In the period leading up to and during WW2 many designers thought that having any more than 12 cylinders on one crank was too many.

Thus Napier designed a series of twin crank engines - the H-16 Rapier, the H-24 Dagger and the H-24 Sabre. The Nomad Mk I was also an H-24, but Diesel.

In the mid '30s Allison was asked to develop an X-24 based on the cylinders of the V-1710. It never passed the mockup stage, Allison instead pushing for the double vee V-3420, which was basically two coupled V-1710s on a common crankcase. The X-24 mockup had the two centre cylinder banks 90°, with the outer pairs the same 60° as the V-1710. Estimated performance was 1600hp with maximum speed of 2600rpm. The V-3420 had the two cranks geared together, rotating in teh same drection on A series engines and contra rotating on B series engines. Some accesories were driven from one crank, while the supercharger and other accessories were driven from th other crank.

Daimler-Benz coupled their engines at the output gearbox. Each half of the engine was a complete self contained unit, complete with all accessories and supercharger, etc. If one half failed it could be disconnected and the other could power the output shaft by itself.

Fairey built prototype H-16 and H-24 engines, the cylinder banks mounted vertically on a common crankcase. In this instance the two halves oerated completely independently, each driving one half of a contra-prop. Rolls-Royce also proposed a similar H-24 version of the Merlin 61.

Rolls-Royce built late in WW2 two engines of about 2800ci - the Eagle 22 (5.4" bore x 5.125" stroke) was a liquid cooled H-24 with two geared together crankshafts, and the air-cooled X-24 Pennine (5.4" bore x 5.08" stroke). Both used sleeve valves.

The question is: which is better - single crank with master and slave rods, a coupled engine or a geared twin crank engine?

Comapring the Pennine and the Eagle 22. The Pennine was 37.5" high x 39" wide and was 106in long. It weighed 2850lb/1293kg and was rated at 2800hp @ 3500 combat rating. The Eagle 22 was 50" high x 43.4" wide and 135" long. The Eagle 22 weighed 3,900 lbs/1,769 kg and was rated at 3500hp @ 3200rpm (depending on source - Wiki says 3200hp @ 3500rpm). Granted, some of the extra length of the Eagle 22 was due to the two stage supercharging and the contra-rotating prop shafts.

The Sabre was 46" high x 40" wide and 82.25" long. Weight was about 2400lb/1088kg, some versions more. Rated power varied oer the years, from 2000hp to over 3000hp, max rpm 4000.

The Vulture was 42.175" high x 35.8" wide x 87.625" long. Weight was 2450lb/1111kg, but had it continued the existing reduction gear would have been changed for an epicyclic system, which sould have saved some weight - about 200lbs/90kg.

Having just typed that I discovered I had received a new e-mail from the RRHT.

The details he provided were:The full throttle heights for the Vulture were 4000ft in Moderate Supercharge (MS) gear and 13,500ft in Full Supercharge (FS) gear. The gear ratios for the supercharger were 5.5 (MS) and 7.3 (FS).

A lot of added complication and manufacturing time and costs when developing a more powerful V12 than the Merlin (Griffon) instead was probably (rightly) seen as the best way to go and good enough for the job at the time.

A lot of added complication and manufacturing time and costs when developing a more powerful V12 than the Merlin (Griffon) instead was probably (rightly) seen as the best way to go and good enough for the job at the time.

But also limited.

With the bigger bore and longer stroke the Griffon was restricted in the boost it could run (eg when the Merlin was cleared for +30psi boost late in the war the Griffon was allowed +25psi) as was the rpm (max rpm for a Griffon was 2750rpm, Merlin normal was 3000rpm but had run to 3300rpm on the test bench).

The Griffon was never rated much more than 2500hp, and most war time models were 2000-2200hp depending on fuel. The Sabre, which was almost identical in displacement, was rated at 2000hp before the war, and over 3000hp by the end of the war, running much lower boost levels.

With the bigger bore and longer stroke the Griffon was restricted in the boost it could run (eg when the Merlin was cleared for +30psi boost late in the war the Griffon was allowed +25psi) as was the rpm (max rpm for a Griffon was 2750rpm, Merlin normal was 3000rpm but had run to 3300rpm on the test bench).

The Griffon was never rated much more than 2500hp, and most war time models were 2000-2200hp depending on fuel. The Sabre, which was almost identical in displacement, was rated at 2000hp before the war, and over 3000hp by the end of the war, running much lower boost levels.

Maybe but much higher revs so probably more stress.

But taken to their logical conclusions it was the Supermarine Spiteful XV1 versus any Sabre engined Tempest bearing in mind the Griffon's advantage of less complexity,less to go wrong,easier/cheaper to produce and maintain.It's easy to understand Rolls' thinking at the time and if I was making the choice between the two for that pylon racing it would have been the Spiteful for me thanks.

Bit like the battle between the good old fashioned simple V8 Cobras and the Ferraris at Le Mans.

But taken to their logical conclusions it was the Supermarine Spiteful XV1 versus any Sabre engined Tempest bearing in mind the Griffon's advantage of less complexity,less to go wrong,easier/cheaper to produce and maintain.It's easy to understand Rolls' thinking at the time and if I was making the choice between the two for that pylon racing it would have been the Spiteful for me thanks.

Bit like the battle between the good old fashioned simple V8 Cobras and the Ferraris at Le Mans.

At the end of the war Supermarine were proposing the Type 391. It had the same or similar wings to the Spiteful, but with leading edge radiators and a Rolls-Royce Eagle 22. Estimated performance was maximum speed 546mph.

Yes, I always wondered what a Sabre with 2-stage supercharging might have done. Unfortunately they were too complex to resurrect and develop for hydroplanes and pylon racers.

Napiers were fiddling with 2 stage 3 speed superchargers before they had entirely solved the reliable production issue. It is part of why English Electric was asked to take over Napier. EE dropped the extra projects and concentrated on getting the basic engine right, and with some (reluctant) help from Bristol they did.

I have always wondered what might have happened if George Mead of Pratt & Whitney had made an agreement for licence production of the Sabre instead of trying to design and build his own.

Looks like the same prop setting Howard Hughes had on his XF-11 during its maiden flight....

The Spiteful and the Type 391 both had the laminar flow wing, which is why they were straight.

The Tempest was a development of the Typhoon with the thinner wing, it ending up with the elliptical wing plan form. Sydney Camm famously said something along the lines of "now we have an elliptical eing they'll have to order it".

I was surprised to read that the Merlin had vertical valves - you would think that angled valves/pentroof etc. would be much better. Was the Griffon the same?

One of my favourite piston-engined fighters is the de Havilland Hornet - probably the best ever.

Yes, the Griffon had the same style combustion chamber.

The original PV12/Merlin had what Rolls-Royce described as ramp heads. The exhaust valve was vertical but the inlets were angled 45° from the vertical - like a pent roof chamber tilted 22.5°. The ramp head had shown promise in single cylinder development, but didn't translate to the full engines.

Ramp heads were on all the pre-production versions and the first production version, the I. They had two piece blocks - that is they had a detachable head.

For the II the cylinder block was redesigned with parallel vertical valves in an integral head. Basically the head arrangement was a scaled up version of the Kestrel head. Issues with getting a good seal between the block and the liner continued until it was replaced with the two piece head in 1941/42. Packard production introduced the two piece head, which many take for an improvement that they had made to the design. But that is not the case - RR had designed the change, but could not implement it in production until they changed over to the two stage engine production. As was often the case in WW2, production won out over improvement.

The Kestrel, Buzzard (6/5 version of Kestrel), R (modified racing Buzzard), Vulture, Peregrine (modernised Kestrel) and Merlin all used the vertical valve heads. Earlier engines, like the Eagle and Condor, did not use the vertical valve layout. The valve layout was borrowed from the Curtiss D-12, with which the Kestrel was to compete (ie make importing/licence production in the UK unecessary).

Shorter strokes and smaller bores allow higher revs with the same stress.

Stress increases at the square of engine speed which in the real world reduces the advantages of using the idea of more smaller capacity cylinders versus less larger capacity ones added to which you've got all that extra complexity of an X or H 24 to effectively end up with more or less a plane with the same performance at the end of the day.So it's still the Spiteful for me thanks (and a Cobra with a nice simple,low revving, big block,pushrod V8 to work on instead of a complex screaming V12 Ferrari ).Although having said that a large capacity V12 sounds good and provides the best of all worlds just as Rolls decided when they ditched the Vulture.If only Jaguar's engineers had designed that 6.0 Litre V12 in my Jag as a pushrod motor instead of all that extra complication of overhead cams though and left the aircraft engineering where it belongs.

One interesting detail on the Merlin is the two rocker pivot shafts each side of the main camshaft. They obviously hold the inner ends of the rockers but they are actually driven by thin straight cut gears off the main cam drive. This allows them to run auxilaries at the other end of the cambox but it also rotates the rocker shaft so evening out wear.

Quite a neat design to save weight/performance impacting wear wear once you acecpt that the whole thing gets rebuilt at a fixed hours limit.

The accessories were driven off the rear end of the engine through from the cam drive or supercharger drive gear trains. No accessories were at the front end of teh cam box on Merlins. The propellor pitch control mechanism was driven off the reduction gear at the front.

In the Griffon the accessories were all mounted and driven off the front of the engine. Early versions even used a flexible drive shaft from the front gear train to drive the supercharger at the rear. Later Griffons replaced this with a short flexible shaft drive from the rear of the crank.

The Vulture used 4 back gears for the propellor reduction drive. These were also used to drive the vertical shaft drive for the cams as well as some of the accessories.

An interesting point of the Rolls-Royce V-12 engines is that they did not employ torsional vibration dampers. They relied on flexible shafts and (for teh supercharger drive) slipping clutches.

However you look at the different ideas it was Rolls' idea to just put all it's eggs in the one basket of continuous development of the Merlin and Griffon V12's that helped to win the war and things might have turned out very differently if they'd have given valuable production and maintenance capacity over to more complicated,more difficult to produce,24 cylinder engines which were probably on the limits of the laws of diminishing returns.

As Goering said it was when he saw the Mustangs over Berlin that he knew the war was lost not Tempests.

The problem was that Merlins and Griffons weren't powerful enough for some aircraft.

The Manchester was a twin, and in 1937 when it was designed 4 Merlins probably wouldn't have been to much better than twin Vultures (other than having better single engine out performance). When the Lancaster was designed and built in 1941 the Merlin it used was at least one and a half times as powerful as the Merlin of 1937. Predicting that would have been difficult. Also remember that the Griffon didn't exit, even on paper, until 1938/39.

As the RAF didn't bomb by daylight they had very little use for a long range single engined fighter. So the Typhoon/Tempest weren't required to travel to Berlin. They were, however, very effective ground support fighters, and very fast at low to medium altitudes.

There were a lot of British projects schemed around the Sabre - such as a larger Mosquito (6000lb bomb load, est 430mph top speed), an unarmed Hawker bomber (the P.1005) and several fighters. But they didn't get anywhere because of the difficulty Napier was having with production of the sleeve valves.

Rolls-Royce's last piston engine projects for WW2 were the Eagle 22 H24 and the Pennine X24, both of around 2800ci.

The problem was that Merlins and Griffons weren't powerful enough for some aircraft.

The Manchester was a twin, and in 1937 when it was designed 4 Merlins probably wouldn't have been to much better than twin Vultures (other than having better single engine out performance). When the Lancaster was designed and built in 1941 the Merlin it used was at least one and a half times as powerful as the Merlin of 1937. Predicting that would have been difficult. Also remember that the Griffon didn't exit, even on paper, until 1938/39.

As the RAF didn't bomb by daylight they had very little use for a long range single engined fighter. So the Typhoon/Tempest weren't required to travel to Berlin. They were, however, very effective ground support fighters, and very fast at low to medium altitudes.

There were a lot of British projects schemed around the Sabre - such as a larger Mosquito (6000lb bomb load, est 430mph top speed), an unarmed Hawker bomber (the P.1005) and several fighters. But they didn't get anywhere because of the difficulty Napier was having with production of the sleeve valves.

Rolls-Royce's last piston engine projects for WW2 were the Eagle 22 H24 and the Pennine X24, both of around 2800ci.

It was probably 'because' of the fact that Rolls didn't put too much of it's development capacity into production of 24 cylinder motors and decided to just concentrate on development of the proven V12 designs that made planes like the Lancaster what they were.It wasn't a case of prediction more development and reliability of the V12 engines keeping pace with,and leading in front of,the demands of the operators requirements.The Lancaster was probably one of,if not the,best bombers of WW2,in addition to that there was the Mosquito, Spitfire Mk 1X and following Griffon versions and the Mustang all of which were on top of their job by combining simplicity,reliability,performance and production numbers.Large scale development of 24 cylinder engines really would have made more of a negative impact, on the allies war effort than what Rolls decided to do considering their main opposition which never got much better than the Focke Wulf 190 Dora 9 with the exception of the Me 262 which even a Griffon Spit or a Tempest couldn't catch unless it was taking off or landing.

In the Dambusters book by Paul Brickhill it said that the Lancs for the Dams raid they got more powerful engines[All Merlins?] from other squadrons. Mostly to get those big bombs off the ground.
Ferrying engines all over the country would have been quite an exercise.
I read somewhere that the different versions had different blowers and a fair bit different specifications.
In that pic above that really looks like a roller cam profile, though it does not seem to have a great deal of overlap. probably helpfull with a blown engine!!

If RR could have continued development of the Vulture they would have - but since it was only used in one production type and the Merlin was needed for many aircraft (most notably the Spitfire and Hurricane). The Lancaster would have taken longer to develop had Rolls-Royce not developed the engine module for the Beaufighter, which was to provide an alternative engine supply in case Bristol could not make enough Hercules.

At the height of teh Battle of Britain production was halted on all but a few key aircraft. Development on the Griffon was even suspended. In fact, I don't think the Griffon would have been placed ahead of the Vulture had it not ben suggested fitting it to the Spitfire.

In the Dambusters book by Paul Brickhill it said that the Lancs for the Dams raid they got more powerful engines[All Merlins?] from other squadrons. Mostly to get those big bombs off the ground. Ferrying engines all over the country would have been quite an exercise.I read somewhere that the different versions had different blowers and a fair bit different specifications.In that pic above that really looks like a roller cam profile, though it does not seem to have a great deal of overlap. probably helpfull with a blown engine!!

Merlins came with single stage single speed superchargers, single stage two speed superchargers, and two stage two speed superchargers.

The sizes of the supercharger impellers also changed. And they were improved in design by Sir Stanley Hooker from 1939 until he moved into jet development.

Different marks of Merlin would often only be different to the next mark by the supercharger gear ratios, and their full throttle heights - the altitude at which the throttle could be opened fully and the maximum power obtained.

Lancasters used Merlin 20 series engines - single stage two speed engines. It is possible that the Dambusters had lower altitude rated engines fitted - as the Dams raid was a low level operation.

The bouncing bomb "Upkeep" wasn't a particularly heavy load for a Lancaster, being under 10,000lb - maximum normal load being 14,000lb. The bombers also had their upper turrets removed.

As fasr as valve timing goes, I don't have the numbers, but there was a page on the web that had them, but I can't find it. Bear in mind that generally the engine operated at a constant rpm - most of the time at cruise settings of around 2600rpm, though they could use as low as 2000rpm, and then will move them up to 2800-2850rpm for short periods (up to about 30 minutes) and then 3000rpm for short dashes at maximum power.

Merlins came with single stage single speed superchargers, single stage two speed superchargers, and two stage two speed superchargers.

The sizes of the supercharger impellers also changed. And they were improved in design by Sir Stanley Hooker from 1939 until he moved into jet development.

Different marks of Merlin would often only be different to the next mark by the supercharger gear ratios, and their full throttle heights - the altitude at which the throttle could be opened fully and the maximum power obtained.

Lancasters used Merlin 20 series engines - single stage two speed engines. It is possible that the Dambusters had lower altitude rated engines fitted - as the Dams raid was a low level operation.

The bouncing bomb "Upkeep" wasn't a particularly heavy load for a Lancaster, being under 10,000lb - maximum normal load being 14,000lb. The bombers also had their upper turrets removed.

As fasr as valve timing goes, I don't have the numbers, but there was a page on the web that had them, but I can't find it. Bear in mind that generally the engine operated at a constant rpm - most of the time at cruise settings of around 2600rpm, though they could use as low as 2000rpm, and then will move them up to 2800-2850rpm for short periods (up to about 30 minutes) and then 3000rpm for short dashes at maximum power.

I couldn't find any numbers either - but I have seen them somewhere. I seem to remember that because of the aero engine's different needs (compared to a car engine) that the point of maximum torque and maximum (allowable) RPM were much the same. Some engines meant for ultralights are also like this. I seem to recall that one German wartime aero engine had a very long duration - to the extent that it was hard to manage.

Allison V-1710-39 was installed in the first 770 production P-51s. Cam Timing: In 48-62: Ex 76-26 Valve lift was 0.533" Valve clearance (cold); In 0.015"; Ex0.020"

The above info from Gruenhagen's book on the P-51. WPT

The Allison's timing is about what I would expect. This is more duration than a car engine could comfortably use on the road. The Merlin timing seems a bit mild. I am surprised to see the figures quoted to two decimal places - seems a bit more accurate than they could have been manufactured.

Not at all. That is the lobe profile a rocker arm engine with an overhead camshaft would need to have for a symmetrical lift profile. WPT

The cam lobe profile does need to be asymmetrical but not to this extent. That is about the biggest "inverse" flank curve I have seen. I think the V12 Phantom car engines also were a bit inverse - in fact quite a few performance engines from the 1930s were inverse. Only really seen these days on radical cams for V-8 dragsters etc.

One of the fundamental problems with the Vulture, early Merlins, etc. can be seen in the pictures you posted of the Merlin cylinder head. Rolls-Royce (and others) did not fully understand the basics of combustion chamber design and detonation. Harry Ricardo in the UK and Luke Hobbs in the US quickly worked through these issues.

As for supercharging, Stanley Hooker made rapid improvements to supercharger design in the UK. And Dr. Sanford Moss made rapid improvements to turbocharger design in the US.

The whole issue of which approach was better for piston aircraft engines- a large number of small displacement, liquid-cooled cylinders, operating at high rpm, or a lesser number of large displacement, air-cooled cylinders, operating at low rpm. History has shown that for aircraft engines, the large displacement, air-cooled, low rpm approach is best.

Liquid cooled engines were genrally smaller in capacity and thus were more highly strung to get the power. Post WW2 the military quickly switched to gas turbines, while the big piston engines soldiered on in transports. No doubting that in the airliner industry having the larger capacity, lower revving less stressed air cooled engines was a plus.

Also, bear in mind that most of the large capacity air cooled engines were 14 or 18 cylinder engines, and often these were in the same power class as the 12 cylinder liquid cooled engines of much smaller capacity.

As stated previously, at the end of the war Rolls Royce were developing the Pennine - an air-cooled X24. It was rated at 2800hp near the beginning of its development, which is equivalent to the best war time R-2800, which was the same capacity, using a lot of boost (for an air-cooled engine) and copious amounts of ADI. It is nearly as much power as the R-4360 was at the time (ie 3000hp). Also more than the larger R-3350 was rated at the time (ie 2200hp).

Liquid cooled engines were genrally smaller in capacity and thus were more highly strung to get the power. Post WW2 the military quickly switched to gas turbines, while the big piston engines soldiered on in transports. No doubting that in the airliner industry having the larger capacity, lower revving less stressed air cooled engines was a plus.

Also, bear in mind that most of the large capacity air cooled engines were 14 or 18 cylinder engines, and often these were in the same power class as the 12 cylinder liquid cooled engines of much smaller capacity.

As stated previously, at the end of the war Rolls Royce were developing the Pennine - an air-cooled X24. It was rated at 2800hp near the beginning of its development, which is equivalent to the best war time R-2800, which was the same capacity, using a lot of boost (for an air-cooled engine) and copious amounts of ADI. It is nearly as much power as the R-4360 was at the time (ie 3000hp). Also more than the larger R-3350 was rated at the time (ie 2200hp).

I think realistically to get the best combination required in a wartime situation it was a 3 horse race between the Junkers Jumo 213,Merlin/Griffon,and the R 2800.The only advantage that the air cooled radial had was it's better ability to withsatand damage to the engine and especially the cooling system of liquid cooled engines which is well documented in wartime reports of between Mustang or Thunderbolt pilots especially those doing ground attacks.Having said that if something like a Corsair,which was running at over 400 mph in October 1940,and the Thunderbolt,had been available in large numbers to the RAF and French Air Forces before the battle of Britain during the BEF retreat from Belgium and France,it's my bet that WW2 probably would have been over before it had really begun and millions of lives could have been saved.In which case the big radial idea can't have a better epitaph than that.

I think realistically to get the best combination required in a wartime situation it was a 3 horse race between the Junkers Jumo 213,Merlin/Griffon,and the R 2800.The only advantage that the air cooled radial had was it's better ability to withsatand damage to the engine and especially the cooling system of liquid cooled engines which is well documented in wartime reports of between Mustang or Thunderbolt pilots especially those doing ground attacks.Having said that if something like a Corsair,which was running at over 400 mph in October 1940,and the Thunderbolt,had been available in large numbers to the RAF and French Air Forces before the battle of Britain during the BEF retreat from Belgium and France,it's my bet that WW2 probably would have been over before it had really begun and millions of lives could have been saved.In which case the big radial idea can't have a better epitaph than that.

If the RAF and French Air Force had the Griffon engine Spitfire XIV in 1940 the same could be said.

If the RAF and French Air Force had the Griffon engine Spitfire XIV in 1940 the same could be said.

Possibly certainly if it was a case of just fighter roles.But having said that the speed of development in the big radials was ahead during 1939-40 and while the Griffon was still on the drawing board the Corsair and the Thunderbolt were much further along the road to production at the time during the Summer of 1940 and it was more a case of,unlike the Griffon powered Spit,the required power unit to do the job was there but the required aircraft to put it in just missing the time by a lesser degree than the development of the Rolls V12 powered aircraft.Which is why the RAF found it so difficult in 1940 because the V12 powered planes available at that time were in the worst of all worlds situation of being relatively few in number and too closely matched,and even inferior in the case of the Hurricane, to the German opposition.

In that type of comparison it was a case of the Corsair and possibly even the Thunderbolt being just a matter of months too late whereas the Griffon Spit was way outside the production dates both in terms of the required engine and the required aircraft design.In addition to that was the T Bolt's well known abilities at both high level dogfighting and ground attack which the Spit couldn't match and the Corsair's record speaks for itself and what was needed in France and Belgium were planes with those types of abilities.On a comparison of what was available and state of design as it stood during 1939-1940 it was the big radial that was in front of the Rolls V12's and the Sabre/Vulture.It was aircraft design that was lagging behind.

Possibly certainly if it was a case of just fighter roles.But having said that the speed of development in the big radials was ahead during 1939-40 and while the Griffon was still on the drawing board the Corsair and the Thunderbolt were much further along the road to production at the time during the Summer of 1940 and it was more a case of,unlike the Griffon powered Spit,the required power unit to do the job was there but the required aircraft to put it in just missing the time by a lesser degree than the development of the Rolls V12 powered aircraft.Which is why the RAF found it so difficult in 1940 because the V12 powered planes available at that time were in the worst of all worlds situation of being relatively few in number and too closely matched,and even inferior in the case of the Hurricane, to the German opposition.

In that type of comparison it was a case of the Corsair and possibly even the Thunderbolt being just a matter of months too late whereas the Griffon Spit was way outside the production dates both in terms of the required engine and the required aircraft design.In addition to that was the T Bolt's well known abilities at both high level dogfighting and ground attack which the Spit couldn't match and the Corsair's record speaks for itself and what was needed in France and Belgium were planes with those types of abilities.On a comparison of what was available and state of design as it stood during 1939-1940 it was the big radial that was in front of the Rolls V12's and the Sabre/Vulture.It was aircraft design that was lagging behind.

Sorry, but the Vulture program had already been cancelled before production aircraft using R-2800s entered service. The B-26 entered service in 1941, the P-47 in 1942 and the Corsair and Hellcat in 1943. The Griffon design was started in 1939, then redesigned to be more compact so it could fit in the Spitfire. Griffons flew in prototype Spitfires in 1941, production of the Mk XII, one of the aircraft used to combat the Fw190, started in 1942 with it entering operational service in 1943.

Even with the problems it had, and having operational restrictions placed upon it, the Vulture was a match for the R-2800 in terms of power in 1939/1940.

Sorry, but the Vulture program had already been cancelled before production aircraft using R-2800s entered service. The B-26 entered service in 1941, the P-47 in 1942 and the Corsair and Hellcat in 1943. The Griffon design was started in 1939, then redesigned to be more compact so it could fit in the Spitfire. Griffons flew in prototype Spitfires in 1941, production of the Mk XII, one of the aircraft used to combat the Fw190, started in 1942 with it entering operational service in 1943.

Even with the problems it had, and having operational restrictions placed upon it, the Vulture was a match for the R-2800 in terms of power in 1939/1940.

But it was the time lag,between prototype to production stage,I was making the point about.The Corsair was actually flying at the pre production stage in May 1940 and was on record as being flown at 400 mph + in October 1940 with the 2800 being able to produce around 2,000 hp in 1939.Which seems to confirm my idea concerning the fact that it was aircraft development and production that was lagging behind all types of engine development but with the big radial being ahead at that time compared to the Merlin or the Vulture which don't forget had to be de rated because of it's reliablity issues.Which comes back to a good lower revving one being better than an overstressed higher revving one.

In the hypothetical comparison,of what might have been,in 1940,if the Corsair had been in it's development/prototype stages just a matter of months earlier and then put into production much faster,and then very soon after it's first flight then supplied in sufficient numbers to the RAF and the French airforces,in time to meet the German invasion of Belgium and France,instead of mainly Hurricanes, (the story went that Dowding was clever enough to keep the few Spitfires we had out of harm's way because he knew that the the Battle of Britain was about to start?),it's radial power that probably would have won out.

The same could be said of the T Bolt to a lesser degree which really needed even more of a faster start point.However in both cases the engine was available and was superior,at that time,to what Rolls could provide which is what made the Corsair and the T Bolt the great planes that they were and there really can't be much argument that the T Bolt was not a better plane,as a combination of everything needed for a successful wartime product,than the Typhoon and Tempest with either Vulture or the Sabre engine.

In the book the timing figures for the -9 engine were given in degrees and seconds. I converted the 'seconds' to degrees for my post. I would not be suprised to learn that the -9 camshaft is just a -3 camshaft advanced by 5 degrees. WPT

But it was the time lag,between prototype to production stage,I was making the point about.The Corsair was actually flying at the pre production stage in May 1940 and was on record as being flown at 400 mph + in October 1940 with the 2800 being able to produce around 2,000 hp in 1939.Which seems to confirm my idea concerning the fact that it was aircraft development and production that was lagging behind all types of engine development but with the big radial being ahead at that time compared to the Merlin or the Vulture which don't forget had to be de rated because of it's reliablity issues.Which comes back to a good lower revving one being better than an overstressed higher revving one.

In the hypothetical comparison,of what might have been,in 1940,if the Corsair had been in it's development/prototype stages just a matter of months earlier and then put into production much faster,and then very soon after it's first flight then supplied in sufficient numbers to the RAF and the French airforces,in time to meet the German invasion of Belgium and France,instead of mainly Hurricanes, (the story went that Dowding was clever enough to keep the few Spitfires we had out of harm's way because he knew that the the Battle of Britain was about to start?),it's radial power that probably would have won out.

The same could be said of the T Bolt to a lesser degree which really needed even more of a faster start point.However in both cases the engine was available and was superior,at that time,to what Rolls could provide which is what made the Corsair and the T Bolt the great planes that they were and there really can't be much argument that the T Bolt was not a better plane,as a combination of everything needed for a successful wartime product,than the Typhoon and Tempest with either Vulture or the Sabre engine.

The Tempest would wipe the floor with any historic 1940 fighter, and was an excellent ground attack weapon. The Typhoon had problems at high altitudes due to teh lack of a suitable supercharger and it was perhaps the most feared ground attack fighter-bomber that the allies had - P-47 included. The Tornado prototype, with the Vulture, flew at 400mph in a similar time frame to the Corsair. The Typhoon was delayed a little by the Sabre.

I would say that if sufficient numbers of most 1943 standard fighters were available for the Battle of France against the Luftwaffe's 1940 equipment they would achieve aerial superiority and make it impossible for the likes of the Stuka to operate. And attacking armour with fighters would have been easier.

The Tempest would wipe the floor with any historic 1940 fighter, and was an excellent ground attack weapon. The Typhoon had problems at high altitudes due to teh lack of a suitable supercharger and it was perhaps the most feared ground attack fighter-bomber that the allies had - P-47 included. The Tornado prototype, with the Vulture, flew at 400mph in a similar time frame to the Corsair. The Typhoon was delayed a little by the Sabre.

I would say that if sufficient numbers of most 1943 standard fighters were available for the Battle of France against the Luftwaffe's 1940 equipment they would achieve aerial superiority and make it impossible for the likes of the Stuka to operate. And attacking armour with fighters would have been easier.

The Tempest wasn't much use at high altitudes either compared to the Focke Wulf Dora 9 which was it's main adversary.Whereas the T bolt could still climb to 40,000 feet and maintain 433 mph at 30,000 which was a far better match for the Focke Wulf unless it was caught at lower levels in which case all the German pilot had to do was stay high and keep the advantage of height.Whereas the Thunderbolt at least had the ability to climb higher and dive faster.In addition to which the Thunderbolt was still a good ground attack aircraft.

But it seems such a shame that so much suffering could probably have been avoided and Germany stopped at the beginning if only allied aircraft development had been just that slight bit more advanced than it was in 1939-40.It wasn't like the governments hadn't had enough warning by the Luftwaffe's actions in the Spanish civil war.

The Tempest wasn't much use at high altitudes either compared to the Focke Wulf Dora 9 which was it's main adversary.Whereas the T bolt could still climb to 40,000 feet and maintain 433 mph at 30,000 which was a far better match for the Focke Wulf unless it was caught at lower levels in which case all the German pilot had to do was stay high and keep the advantage of height.Whereas the Thunderbolt at least had the ability to climb higher and dive faster.In addition to which the Thunderbolt was still a good ground attack aircraft.

But it seems such a shame that so much suffering could probably have been avoided and Germany stopped at the beginning if only allied aircraft development had been just that slight bit more advanced than it was in 1939-40.It wasn't like the governments hadn't had enough warning by the Luftwaffe's actions in the Spanish civil war.

If the aircraft was more developed in the Allied countries then it would have been also for the Axis.

The problem with having forewarning is having time to do anything about it. The RAF was updating its equipment feverishly in the '30s, but they were still outnumbered by the Luftwaffe. The US was still neutral in 1940, though they were supplying equipment and materials to Britain and France.

I'd still prefer to be in a Spitfire XIV than a Thunderbolt for air to air combat. The T-bolt would be better suited to ground attack.

The high altitude performance of the T-Bolt is mostly due to the turbocharger - the engine worked as if it was at sea level untill about 30,000ft. If the RAF had instructed Hawker to fit one to teh Tempest then it would have been a better high altitude plane also.

If the aircraft was more developed in the Allied countries then it would have been also for the Axis.

I'd still prefer to be in a Spitfire XIV than a Thunderbolt for air to air combat. The T-bolt would be better suited to ground attack.

The issue of wether the Axis could have kept pace with slightly more advanced allied aircraft development is probably more arguable considering the state of engine development for the allies when compared with that of the axis.The Jumo 213 was the only realistic option open to them if the Griffon Spit and the 2800 radial powered planes like the Corsair and T bolt had been available for production in 1939/40.However the Jumo was effectively a non runner at these power outputs until 1943.Which probably would have left the Axis in the position of having the Me 109 and possibly the BMW radial powered Focke Wulf 190 against a force made up of both the French and British airforces equipped with Griffon Spits,Corsairs and T bolts soon to be joined by Griffon powered Mustangs.

So the hypothesis here,the way I see it, seems to have turned from the blind alley of the what if the 24 cylinder engines had been developed to what if it was the Griffon that Rolls concentrated on instead of the Merlin after the first flight of the Spitfire with the obviously underpowered Merlin and what if the Americans had been that bit further along the road with their aircraft design providing something decent to put the 2800 into in 1939 and production of the Mustang with Griffon power not Merlin,sooner.I really wouldn't have wanted to be a Lutwaffe pilot in that scenario.

R-2800 production figures: 1939-2, 1940-17,1941-1733; 1939 thru 1945-122,175. By 1945 five factories were producing the R-2800. The development of this engine and the factories to produce it took some time. In fact some grinding machines that P&W needed to set up their second factory were sent instead to Napier in GB for Sabre production. Ford (of the five factories) built the most R-2800s during the war. The 'C' engine, which used a new crank and cases (from the 'A' and 'B' engines) was developed during the war.

Production figures for the F4U: 1940-1, 1941-0, 1942-178. The prototype that went 406MPH in 1940 crashed in testing setting the program back. It also was nothing like the F4U-1 production item. The F4U-4 did not make it into the war till late 1944.

Production figures for the P-47: 1940-0, 1941-1, 1942-526. So, again, it took some time to develop the plane and build the factories.

What the USA did have in production in 1940 was the P-40, and when GB tried to buy some, were told to check with North American about setting up a second production line. This led directly to the P-51. Know GB ordered P-39s, flew one mission with them, and sent them back, or maybe to Russia. When the Eagle Sqds were moved into the 8th AF in Sep 1942 they had to be equiped with Spitfires till the P-47 made it across in early 1943. It is what it is.

Prototype FW-190D9 used a MB-603 engine. Would this not have been a better engine for the D9 than the Junkers Jumo 213?

R-2800 production figures: 1939-2, 1940-17,1941-1733; 1939 thru 1945-122,175. By 1945 five factories were producing the R-2800. The development of this engine and the factories to produce it took some time. In fact some grinding machines that P&W needed to set up their second factory were sent instead to Napier in GB for Sabre production. Ford (of the five factories) built the most R-2800s during the war. The 'C' engine, which used a new crank and cases (from the 'A' and 'B' engines) was developed during the war.

Production figures for the F4U: 1940-1, 1941-0, 1942-178. The prototype that went 406MPH in 1940 crashed in testing setting the program back. It also was nothing like the F4U-1 production item. The F4U-4 did not make it into the war till late 1944.

Production figures for the P-47: 1940-0, 1941-1, 1942-526. So, again, it took some time to develop the plane and build the factories.

What the USA did have in production in 1940 was the P-40, and when GB tried to buy some, were told to check with North American about setting up a second production line. This led directly to the P-51. Know GB ordered P-39s, flew one mission with them, and sent them back, or maybe to Russia. When the Eagle Sqds were moved into the 8th AF in Sep 1942 they had to be equiped with Spitfires till the P-47 made it across in early 1943. It is what it is.

Prototype FW-190D9 used a MB-603 engine. Would this not have been a better engine for the D9 than the Junkers Jumo 213?

From those figures it seems like America was a lot less prepared for the war that was coming than even Britain was and had allowed it's defence technology to fall way behind where it needed to be at the start of the war.It seemed like the worst of all worlds in that while they were ahead in design with the specification of the 2800 that was then thrown away by the deficiencies in the rate of both engine and aircraft development and production capacity.

From the RAF's point of view it was a case of a war that maybe could have been won at the beginning,given aircraft with the performance of later types of Spitfires like the Mk IX and Mk XIV and good multi role aircraft like the Thunderbolt,whereas it was a case of a war almost lost at the beginning,because of the lack of superiority of the earlier types of aircraft like the less powerful early Marks of Spits and Hurricanes,over their German opposition,at the time of the German invasion of Belgium and France and the Battle of Britain at the beginning.

The issue of Rolls putting resources into the Vulture would only have added to that problem just as Hawkers putting resources into the Hurricane probably did.

History seems to prove that the Jumo 213 was the right engine for the Focke Wulf on the German side.

The British piston aero engine development efforts in the late 1930's and early 1940's definitely had more challenges than those in the US. Some were due to the proximity to the combat, and some were due to less financial and manpower resources being available. But some of the problems were brought about by poor decisions made by the British engine companies themselves. The British had far too many risky engine development programs (ie. sleeve valve designs), which diverted precious resources away from improving the performance and production rates of established designs like the Merlin. The US, on the other hand, tended to be more conservative and focused on getting the most performance from the low-tech, 2 valve, pushrod, air-cooled radial engine (an interesting historical footnote is that the highest quality Merlins produced during the war were those manufactured in the US under license by Packard).

That's not to say that the US air cooled radials were "trouble free". Just like the Vulture, the Pratt R-2800 had its share of development issues (primarily torsional vibration and heat transfer problems). But thankfully, Pratt had a first class engineer in Luke Hobbs overseeing the program. And Hobbs quickly resolved these issue through a methodical and analytically based approach. During the 4 years of the R-2800's war time development, it became the most reliable and effective engine produced by the US.

When it came to the overall compromise of cost, weight, fuel consumption, reliability, damage tolerance, installed drag, etc., I'd still argue that the big displacement, slow revving, air-cooled radial was best.

From those figures it seems like America was a lot less prepared for the war that was coming than even Britain was and had allowed it's defence technology to fall way behind where it needed to be at the start of the war.It seemed like the worst of all worlds in that while they were ahead in design with the specification of the 2800 that was then thrown away by the deficiencies in the rate of both engine and aircraft development and production capacity.

From the RAF's point of view it was a case of a war that maybe could have been won at the beginning,given aircraft with the performance of later types of Spitfires like the Mk IX and Mk XIV and good multi role aircraft like the Thunderbolt,whereas it was a case of a war almost lost at the beginning,because of the lack of superiority of the earlier types of aircraft like the less powerful early Marks of Spits and Hurricanes,over their German opposition,at the time of the German invasion of Belgium and France and the Battle of Britain at the beginning.

The issue of Rolls putting resources into the Vulture would only have added to that problem just as Hawkers putting resources into the Hurricane probably did.

History seems to prove that the Jumo 213 was the right engine for the Focke Wulf on the German side.

No, the US was not prepared for war. In fact, through the 1930s the effects of the depression and a national policy of isolationalism made getting funds for the development of military aircraft difficult.

Add to that the USAAC leadership was dominated by a group of officers often known as the Bomber Barons.

When, in 1937, the Messerschmitt Bf109 and Hawker Hurricane were entering service with their respective air forces the USAAC's primary fighter was the Boeing P-26. The following year the USAAC were updating to P-35s and P-36s and the Spitfire was becoming operational with the RAF.

The RAF was in a modernisation program for much of the 1930s. Britain was expecting war with Germany - not a case of if, but when.

The timing of the Griffon and Merlin were problematic for RR. The Griffon design didn't start until 1939, by which time the Merlin was being used in many different types. With the onset of war the importance of making as many aircraft and engines went to a new level. While the Griffon showed potential it would not be around in sufficient quantities in time, and production of the Merlin was already in full swing, with Packard being lined up as another production facility.

Against that the Vulture was only being used for one production type - the Manchester. And the Vulture was having problems (no more than the Merlin had experienced earlier). Similarly the Peregrine was only in one production machine - the Whirlwind. The Exe was only being used in RR's experimental hack. It was entirely logical to concentrate on teh Merlin.

Two other things influenced the decision to drop the Vulture and continue with the Griffon, IMO. The first was that Rolls-Royce had developed a quick engine change module, or power egg, for the Beaufighter in case the Bristol Hercules production was insufficient. Without these the redesign of the Manchester into the Lancaster may have taken too long. The other was a suggestion from a member of the Ministry of Aircraft Production in late 1939 that the Griffon might be adapted to the Spitfire. This led to a redesign of the Griffon, which enabled it to be fitted into the Spitfire, and potentially any other Merlin powered aircraft.

That's not to say that the US air cooled radials were "trouble free". Just like the Vulture, the Pratt R-2800 had its share of development issues (primarily torsional vibration and heat transfer problems). But thankfully, Pratt had a first class engineer in Luke Hobbs overseeing the program. And Hobbs quickly resolved these issue through a methodical and analytically based approach. During the 4 years of the R-2800's war time development, it became the most reliable and effective engine produced by the US.

And one wonders if the US had been attacked and needed as many P-35s and P-36s, which both used the Pratt & Whitney R-1830 Twin Wasp, for the defence of the country if P&W would have had enough resoureces to improve the R-1830 while solving the R-2800's issues.

When it came to the overall compromise of cost, weight, fuel consumption, reliability, damage tolerance, installed drag, etc., I'd still argue that the big displacement, slow revving, air-cooled radial was best.

Slow revving? - P&W R-2800 - 2800rpm. RR Merlin - 3000rpm.

In terms of weight the radial is ahead on weight/cubic inch. But not necessarily in weight per installed hp. Fuel consumption - yes, radials were generally better, and some better than others - and is one of the reasons airliners preferred air cooled engines. Reliability depends on which engines you are talking about. In 1944 I would take a Merlin over an R-3350 for reliability any day! Damage tolerence is an advantage - more so in the ground attack role.

Installed drag? That is a tough one to substantiate. Generally a fighter with a liquid cooled engine could be as fast, or faster, on lower power. The installed drag of the radial engine was improved greately through the 1930s and WW2. But the NACA cowl, which is where most of the improvement came, could also be applied to liquid cooled engines using an annular radiator - as was used by several German aircraft. And the radial engine has a much larger frontal area.

Also consider propellor efficiency. NACA tested a range of propellors in front of a liquid cooled engine and a radial engine and found that the propellors were as much as 5-7% more efficient in front of the former than they were the latter.

And one wonders if the US had been attacked and needed as many P-35s and P-36s, which both used the Pratt & Whitney R-1830 Twin Wasp, for the defence of the country if P&W would have had enough resoureces to improve the R-1830 while solving the R-2800's issues.

Slow revving? - P&W R-2800 - 2800rpm. RR Merlin - 3000rpm.

In terms of weight the radial is ahead on weight/cubic inch. But not necessarily in weight per installed hp. Fuel consumption - yes, radials were generally better, and some better than others - and is one of the reasons airliners preferred air cooled engines. Reliability depends on which engines you are talking about. In 1944 I would take a Merlin over an R-3350 for reliability any day! Damage tolerence is an advantage - more so in the ground attack role.

Installed drag? That is a tough one to substantiate. Generally a fighter with a liquid cooled engine could be as fast, or faster, on lower power. The installed drag of the radial engine was improved greately through the 1930s and WW2. But the NACA cowl, which is where most of the improvement came, could also be applied to liquid cooled engines using an annular radiator - as was used by several German aircraft. And the radial engine has a much larger frontal area.

Also consider propellor efficiency. NACA tested a range of propellors in front of a liquid cooled engine and a radial engine and found that the propellors were as much as 5-7% more efficient in front of the former than they were the latter.

In the case of the 2800 it was one of the good ones and the comparison for the topic is with the 24 cylinder Napre and/or Sabre.In which case it's performance and reliability seem to have been good enough when fitted in the Corsair and Thunderbolt compared to the Vulture and most of the Sabre applications and even the Tempest was arguably a better plane when fitted with te Bristol Centaurus instead of the Sabre.I'd still stand by the idea that Rolls made the right decision in dropping the idea and the only thing is why did they and the government procurement departments leave it so long.Especially considering that it might have delayed the eventual introduction of the Griffon Spits.